As technology continues to evolve in telecommunications, optics is playing an ever-increasing role and, in many respects, has become predominant over conventional electrical transmissions. An important subsystem of an optical telecommunication system is the optoelectric interface which converts signals between the optical domain and the electrical domain. Typically, the optoelectric interface comprises some kind of optoelectric device (OED) for either transmitting or receiving optical signals. OEDs are commonly packaged in “TO can” assemblies which comprise a cap and a header assembly. The header assembly can be a hybrid microelectronic assembly which includes the OED plus one or more integrated circuits (ICs) or passive components. The header assembly includes an insulating plate, such as a suitable ceramic submount, on which the ICs and OED are mounted. The insulating plate, in turn, is mounted on a metallic header through which leads extend. Therefore, a TO can comprises an assembly of electrical and mechanical elements involving multiple assembly steps and electrical connections.
In addition to the OED, a typical optoelectric interface comprises a printed circuit board or substrate containing the necessary circuitry to operate the OED, and a connector interface for interfacing with an optical connector of an optical fiber or cable (multiple fibers). A typical optical connector comprises a housing with a ferrule disposed therein. The ferrule is configured to hold one or more fibers in a specific spacial relationship and has an end face which presents the fiber ends in an array. The end face is generally polished to provide for good optical contact with a mating face of the module. The combination of the fiber and optical connector is referred to herein as a “cable assembly”.
Generally, the OED, supporting circuitry, substrate and connector interface are packaged into a discrete module, referred to herein as an “opto-electric module” or just “module.” Optoelectric modules are generally configured for installation into larger host systems such as routers and computers. The modules are typically installed on host circuit boards within such host systems using conventional installation configurations such as through-pin mounting or pluggable receptacles. Since the modules are configured to interconnect to cable assemblies, the modules are typically positioned near the perimeter of the host system for accessibility and to avoid the need for circuitously routing the fiber through the host system. The desire to minimize the access area required for each module has lead to modules being elongated and rectilinear in shape such that they present an end face having a relatively small area for connection to the cable assembly.
For purposes of illustration, reference is made to the optoelectric module's orientation with respect to the x,y, and z axes in accordance with the Cartesian coordinate system. Unless otherwise indicated, the z axis is the axis along which light enters the module. Recent trends in module design promote configurations in which light enters essentially parallel to the substrate. Therefore, the z-axis will typically be along the length of the elongated modules and the x,y axes will typically define the area of the end face.
The continuing need for miniaturization in the telecommunications field has impacted optoelectric modules in several respects. First, the need to populate the backplane of host systems with as many modules as possible has intensified the need to reduce the modules' x,y area. Indeed, recent trends in industry standards have seen a precipitous reduction in the x,y area of the module and this trend is likely to continue. Recent modules designs which have reduced x,y areas have been termed “small-form factor” designs.
Complicating the desire for small-form factor designs is a competing desire for increasing the number of fibers presented in a single optical connector. The industry is evolving from single fiber ferrules to multi-fiber ferrules containing two or more fibers arranged in an x,y array. The x,y arrays typically are elongated along the x axis and comprise a plurality of columns along the y axis and one or more rows along the x axis. Examples of commonly-used, multi-fiber connectors include the MT-RJ type of connectors which have a single row of two or more fibers and the Lightray MPX™ line of connectors which have one or more rows of one or more fibers. Therefore, the desire for small form factor designs combined with the desire for multi-fiber arrays has lead to the need for modules cable of handling compact multi-fiber arrays also referred to herein as “high-density fiber arrays.”
Unfortunately, interfacing with high-density fiber arrays has been problematic, especially with conventional OEDs such as TO cans. More specifically, TO cans tend to be bulky and the space between fibers in high-density arrays tends to be insufficient to accommodate their bulk. One approach for accommodating the tight space requirements of high-density arrays is to employ elaborate light reflecting optics to expand the array along the x axis and thereby increase separation between optical paths (see, e.g, Hewlett Packard MT-RJ transceiver m/n HFBR-5903,2). Although such an approach works for relatively simple fiber arrays, for example, a single row of two fibers, this approach tends to be problematic as the number of fibers increases since the room available for expansion along the x-axis becomes less until there is simply insufficient room to accommodate all the TO cans. Additionally, the complex optics needed to expand the distances along the x-axis for even a two fiber array tend to be expensive and problematic from a manufacturing perspective. Such expense and manufacturing difficulty is expected to increase exponentially as the number of fibers in the array increases.
Aside from expense and manufacturing difficulty, optics associated with increasing separation along the x-axis are particularly susceptible to deformation along the optical paths caused by thermal instability encountered during the module's operation. Specifically, such optics typically comprise molded plastic which tends to expand/contract with thermal changes. If the plastic expands/contracts along the direction of parallel optical paths, no distortion is introduced. On the other hand, if the plastic expand/contracts where the optical paths change direction relative to one another, distortion is introduced. Since optics separating optical paths along the x-axis necessarily define divergent optical paths (non-parallel), distortion will be introduced during expansion/contraction. Additionally, any change in the deformation of the optical assembly imposed during assembly and will tend to affect the divergent optical paths differently. For example, if the optical assembly is twisted lengthwise, one divergent path will be deformed upward while the other path will be deformed downward. Furthermore, the reflective surfaces in the optics are separated by relatively large distances and, thus, any deformation of the optics will tend to be magnified by the distance.
Aside from limiting the number of fibers in dense-fiber arrays, TO cans present other problems which make them undesirable. For example, the cylindrical shape makes them difficult to handle using automated pick and place machinery, and, thus, they are usually integrated into modules by hand which is time-consuming and expensive. Their shape also requires active alignment along the x,y,z axes since a cylinder has no reference surface.
Aside from shortcomings associated with their shape and size, TO cans also tend to have imprecise feedback control. More specifically, a traditional TO can typically samples only a section of light being generated by the semi-conductor contained within the can. Consequently, the average power of the transmitted beam can only be approximated from the reflected section. Recently, TO cans with angled windows have been introduced which reflect a portion of the entire beam, but such a configuration requires that very precise alignment of the chip on the header. This adds additional complexity to the manufacture of the TO can, which, in turn, drives up the cost.
Therefore a need exists for a small form factor optical module that can accommodate high-density fiber arrays without complex, error-prone optics and without using traditional TO cans and experiencing the problems associated therewith. The present invention fulfills this need among others.